The GIN interface has a
high level of abstraction, requiring the access method
implementer only to implement the semantics of the data type
being accessed. The GIN layer
itself takes care of concurrency, logging and searching the tree
structure.

All it takes to get a GIN
access method working is to implement four (or five) user-defined
methods, which define the behavior of keys in the tree and the
relationships between keys, indexed items, and indexable queries.
In short, GIN combines
extensibility with generality, code reuse, and a clean
interface.

The four methods that an operator class for GIN must provide are:

int compare(Datum a, Datum
b)

Compares two keys (not indexed items!) and returns an
integer less than zero, zero, or greater than zero,
indicating whether the first key is less than, equal to, or
greater than the second. Null keys are never passed to this
function.

Datum *extractValue(Datum
itemValue, int32 *nkeys, bool **nullFlags)

Returns a palloc'd array of keys given an item to be
indexed. The number of returned keys must be stored into
*nkeys. If any of the keys can be
null, also palloc an array of *nkeysbool fields,
store its address at *nullFlags,
and set these null flags as needed. *nullFlags can be left NULL (its initial value) if all keys are
non-null. The return value can be NULL if the item contains no keys.

Returns a palloc'd array of keys given a value to be
queried; that is, query is the
value on the right-hand side of an indexable operator whose
left-hand side is the indexed column. n is the strategy number of the operator
within the operator class (see Section 35.14.2).
Often, extractQuery will need
to consult n to determine the data
type of query and the method it
should use to extract key values. The number of returned
keys must be stored into *nkeys.
If any of the keys can be null, also palloc an array of
*nkeysbool
fields, store its address at *nullFlags, and set these null flags as
needed. *nullFlags can be left
NULL (its initial value) if all
keys are non-null. The return value can be NULL if the query
contains no keys.

searchMode is an output
argument that allows extractQuery to specify details about how
the search will be done. If *searchMode is set to GIN_SEARCH_MODE_DEFAULT (which is the value
it is initialized to before call), only items that match at
least one of the returned keys are considered candidate
matches. If *searchMode is set to
GIN_SEARCH_MODE_INCLUDE_EMPTY,
then in addition to items containing at least one matching
key, items that contain no keys at all are considered
candidate matches. (This mode is useful for implementing
is-subset-of operators, for example.) If *searchMode is set to GIN_SEARCH_MODE_ALL, then all non-null items
in the index are considered candidate matches, whether they
match any of the returned keys or not. (This mode is much
slower than the other two choices, since it requires
scanning essentially the entire index, but it may be
necessary to implement corner cases correctly. An operator
that needs this mode in most cases is probably not a good
candidate for a GIN operator class.) The symbols to use for
setting this mode are defined in access/gin.h.

pmatch is an output argument
for use when partial match is supported. To use it,
extractQuery must allocate an
array of *nkeys booleans and store
its address at *pmatch. Each
element of the array should be set to TRUE if the
corresponding key requires partial match, FALSE if not. If
*pmatch is set to NULL then GIN assumes partial match is not
required. The variable is initialized to NULL before call, so this argument can simply
be ignored by operator classes that do not support partial
match.

extra_data is an output
argument that allows extractQuery to pass additional data to
the consistent and
comparePartial methods. To
use it, extractQuery must
allocate an array of *nkeys
Pointers and store its address at *extra_data, then store whatever it wants to
into the individual pointers. The variable is initialized
to NULL before call, so this
argument can simply be ignored by operator classes that do
not require extra data. If *extra_data is set, the whole array is
passed to the consistent
method, and the appropriate element to the comparePartial method.

Returns TRUE if an indexed item satisfies the query
operator with strategy number n
(or might satisfy it, if the recheck indication is
returned). This function does not have direct access to the
indexed item's value, since GIN does not store items explicitly.
Rather, what is available is knowledge about which key
values extracted from the query appear in a given indexed
item. The check array has length
nkeys, which is the same as the
number of keys previously returned by extractQuery for this query datum. Each element of the check array is TRUE if the indexed item
contains the corresponding query key, ie, if (check[i] ==
TRUE) the i-th key of the extractQuery result array is present in
the indexed item. The original query datum is passed in case the
consistent method needs to
consult it, and so are the queryKeys[] and nullFlags[] arrays previously returned by
extractQuery. extra_data is the extra-data array returned
by extractQuery, or
NULL if none.

When extractQuery returns
a null key in queryKeys[], the
corresponding check[] element is
TRUE if the indexed item contains a null key; that is, the
semantics of check[] are like
IS NOT DISTINCT FROM. The
consistent function can
examine the corresponding nullFlags[] element if it needs to tell the
difference between a regular value match and a null
match.

On success, *recheck should be
set to TRUE if the heap tuple needs to be rechecked against
the query operator, or FALSE if the index test is exact.
That is, a FALSE return value guarantees that the heap
tuple does not match the query; a TRUE return value with
*recheck set to FALSE guarantees
that the heap tuple does match the query; and a TRUE return
value with *recheck set to TRUE
means that the heap tuple might match the query, so it
needs to be fetched and rechecked by evaluating the query
operator directly against the originally indexed item.

Compare a partial-match query key to an index key.
Returns an integer whose sign indicates the result: less
than zero means the index key does not match the query, but
the index scan should continue; zero means that the index
key does match the query; greater than zero indicates that
the index scan should stop because no more matches are
possible. The strategy number n of
the operator that generated the partial match query is
provided, in case its semantics are needed to determine
when to end the scan. Also, extra_data is the corresponding element of
the extra-data array made by extractQuery, or NULL if none. Null keys are never passed to
this function.

To support "partial match" queries,
an operator class must provide the comparePartial method, and its extractQuery method must set the pmatch parameter when a partial-match query is
encountered. See Section 57.3.2
for details.

The actual data types of the various Datum values mentioned above vary depending on the
operator class. The item values passed to extractValue are always of the operator class's
input type, and all key values must be of the class's STORAGE type. The type of the query argument passed to extractQuery and consistent is whatever is specified as the
right-hand input type of the class member operator identified by
the strategy number. This need not be the same as the item type,
so long as key values of the correct type can be extracted from
it.